A fructosyl amino acid oxidase catalyzes an action to generate glyoxylic acid or α-ketoaldehyde, α-amino acid and hydrogen peroxide by oxidizing iminodiacetic acid or a derivative thereof (also referred to as the “Amadori compound”) in the presence of oxygen. The fructosyl amino acid oxidase was discovered in fungi and bacteria. Analytical methods using these enzymes for measuring the Amadori compound included in foods and generated in vivo have been proposed (JP Patent Publication (Kokoku) No. 5-33997, JP Patent Publication (Kokoku) No. 6-65300, JP Patent Publication (Kokai) No. 2-195900, JP Patent Publication (Kokai) No. 3-155780, JP Patent Publication (Kokai) No. 4-4874, JP Patent Publication (Kokai) No. 5-192193, and JP Patent Publication (Kokai) No. 6-4686).
Recently in the field of clinical diagnosis, HbA1c (glycosylated hemoglobin) has attracted attention as a blood glucose control marker, which is important in diagnosing the pathological conditions and controlling the symptoms of diabetic patients. As a method for rapidly and simply measuring HbA1c, an enzymatic measurement method using fructosyl amino acid oxidase has been proposed and practically applied. Specifically, this method involves degrading HbA1c with protease or the like, and then measuring the thus-freed fructosyl valine (a derivative of iminodiacetic acid wherein α-amino group of valine is glycosylated). Among various fructosyl amino acid oxidases that are used as enzymes in this measurement method, an enzyme having high activity for fructosyl valine, which is a substrate, but having no activity for ε-fructosyl lysine (a type of Amadori compound wherein ε-amino group of lysine is glycosylated) is particularly excellent in terms of its rapidity and accuracy of measurement. An example of such an enzyme is fructosyl amino acid oxidase derived from bacteria of the genus Corynebacterium. However, the above fructosyl amino acid oxidase is deactivated 90% or more by heat treatment at 45° C. for 10 minutes. Because of this low heat resistance, the fructosyl amino acid oxidase has a problem in its storage stability when it is prescribed in a kit reagent as an enzyme for clinical diagnosis.